Piezoelectric resonator utilizing electrodes larger than the polarized region for controlling the coupling coefficient thereof

ABSTRACT

Only part of the center of a ceramic body, such as a circular disc, is subjected to a polarizing voltage. In the case of a radial mode resonator this poled area may be a circular area in the center of the major faces of the resonator. This poling is accomplished by using temporary electrodes, which may or may not become part of the final electrode structure. After the element has been thus poled, a new or added electrode is applied to the resonator. This electrode is greater in area than the area of the poling electrode, and can be placed immediately over the poling electrodes, while still making electrical contact to the adjacent area of the ceramic body, or to the poling electrode, which in turn is in contact to the ceramic. By controlling the percentage area of the final electrode that is poled it is possible to decrease the separation of the resonant and antiresonant frequencies of the piezoelectric resonator, thereby decreasing the coupling factor of the element.

United States I Patent I 72] Inventor Ralph K. Hell'en Chicago, Ill.

[21] Appl. No 801.118

[22] Filed Feb. 20, I969 {45] Patented June 8,1971

[73] Assignee P. R. Mallory & Co., Inc.

Indianapolis, Ind.

[54] PIEZO-ELECTRIC RESONATOR UTILIZING ELECTRODES LARGER THAN THEPOLARIZED REGION FOR CONTROLLING THE COUPLING COEFFICIENT THEREOF 4Claims, 8 Drawing Figs.

[52] U.S. Cl 3l0/9.5,

- 31018.9, 3l0/9.4 [-51] Int. Cl H01v 7/00 [50] Field of Search 310/9.7,

[56] References Cited UNITED STATES PATENTS 2,640,165 5/1953 Howatt310/9.7X

2,974,296 3/1961 Rosen 310/8.2X

3,017,525 1/1962 Wolfskill 310/9.7X

3,104,377 9/1963 Alexander et a1... 310/9.7X

3,374,367 3/1968 Cowan 310/9.7X

3,384,768 5/1968 Shockley 31019.5 3,401,283 9/1968 Curran et al....310/9.7X 3,423,700 1/1969 Curran et al. 3 l0/9.7X 3,433,982 3/1969Kaname et al. 310/9.7X

8 ABSTRACT: Only part of the center of a ceramic body, such as acircular disc, is subjected to a polarizing voltage. In the case of aradial mode resonator this poled area may be a circular area in thecenter of the major faces of the resonator. This poling is accomplishedby using temporary electrodes, which may or may not become part of thefinal electrode structure. After the element has been thus poled, a newor added electrode is applied to the resonator. This electrode isgreater in area than the area of the poling electrode, and can be placedimmediately over the poling electrodes, while still making electricalcontact to the adjacent area of the ceramic body, or to the polingelectrode, which in turn is in contact to the ceramic. By controllingthe percentage area of the final electrode that is poled it is possibleto decrease the separation of the resonant and antiresonant frequenciesof the piezoelectric resonator, thereby decreasing the coupling factorof the element.

PATENTEUJUN 8l97l 3584.245

' SHEET 2 [1F 2 ATTORNEY PIEZO-ELECTRIC RESONATOR UTILIZING ELECTRODESLARGER THAN THE POLARIZED REGION FOR CONTROLLING THE COUPLINGCOEFFICIENT THEREOF DESCRIPTION This invention relates to peizoelectricresonators. lt specifically relates to a method of preparing apiezoelectric resonator and its construction, to provide for controllingthe coupling coefficient of the resonator when finally completed. Suchcoupling characterizes the frequency range of operation of theresonator. Thus, the relationship between the physical characteristicsand the parameters of a piezoelectric resonator element, such as a plateof disc, determines the electrical characteristics or parameters, suchas the mass, the compliance, and the loading or energy dissipation,which collectively govern and control the natural frequency output ofthe piezoelectric element. That output frequency is the ultimateparameter, or output, desired from the resonator for appropriate controlof an associated electrical circuit.

In the prior art, the coupling or frequency characteristic of apiezoelectric resonator has been varied by altering the composition ofthe material employed to constitute the resonator element, or byaltering the processes associated with the manufacturing of thosematerials in producing the resonator element. Such prior art practiceand techniques required different material compositions for applicationswhich require different coupling factors.

The bandwidth of a piezoelectric resonator, when used individually as abypass element or in combinations as in a filter array, is proportionalto a coupling coefficient of the device. This coupling is determined bythe separation of the resonant and antiresonant frequencies. For a givenresonant frequency, the greater separation of the two characteristicfrequencies indicates an increase in coupling.

Previous methods to alter the resonatorcoupling factor either decreasedthe mechanical Q of the vibrator, thereby (1) increasing losses, or (2)utilized a process which demanded a decrease in the polarization field,and was very difficult to control.

The process of adjusting the coupling factor of a resonator, under thisinvention, used full poling strength, which reduces the tendency tochange the poling of the object once it is installed in a circuit andsubjected to a voltage. Parts constructed in accordance with thisinvention, can be subjected to higher potentials without affecting theoriginal coupling factor or bandwidth characteristics of the resonatoror filter.

In accordance with the present invention, a piezoelectric element,illustrated in a preferred form as a circular disc in the presentdescription, is polarized in the central coaxial region of the disc toestablish two polar areas coaxially aligned and spaced by the thicknessof the disc or plate, which is ordinarily quite uniform. After thispolarized region is established, two electrodes are applied coaxially tothe opposite faces of the disc, over an area that is larger in diameterthan the treated area which was polarized. A small annular border areaon each of the opposite major faces of the resonator is left exposed topermit a greater freedom to be available at the border and peripheraledge of the disc during vibrational operation at the operating frequencyof the resonating plate or disc.

A primary object of the invention is to provide a method of preparingand conditioning a piezoelectric resonator disc or plate, in order tocompensate for batch variations where the plates are manufactured frommaterials taken from different batches or mixtures, or to compensate forthe use of materials from the same batch, and are to be applied in avariety of applications, where, normally, materials from differentmixtures with different coupling characteristics would be required.

Another object of the invention is to provide a method of manufacturingand treating a resonator plate or disc, to modify the electricalparameters, and thereby to modify the coupling factor and the range ofoperating frequency of the resonator disc.

Another object of the invention is to provide a method of manufactureand treatment of the resonator disc that will artificially increase theeffective capacitance parameter of the piezoelectric disc, in orderthereby to modify and control and reduce the operating antiresonantfrequency of the disc, and thereby shift such operating antiresonantfrequency closer to the natural resonant frequency of the disc, with aresultant reduction in the coupling factor and in the range of frequencyoperation of the resonator disc. As a result, the tuning band will benarrower and sharper and permit finer control response and reactionbetween the resonator and the associated circuits with which it may becombined.

A particularly important feature of this invention is that it permitsthe detection of poor dielectric material, or of a poor sample of thedielectric material, during pretreatment, that is unsuitable as aresonator. By such early detection of a poor sample of material thatwill be unsuitable for an ultimate high quality resonator, the costlyexpense of putting final electrodes on these poor material wasteelements is thereby eliminated.

Thus, another object of the invention is to provide as part of theconditioning process of the invention, a conditioning step, which, inthe treatment of the disc, detects a poor material or inadequate sampleat an early stage in the preparation of the sample for manufacture as aresonator.

Thus, for example, where a sample is to be polarized to operate at acertain predetermined frequency range, the polarizing step at highvoltage would cause a breakdown in a poor sample and thus wouldeliminate that sample from further attention in making a resonator, andparticularly would eliminate the expense of the costly step in theoperation of applying finished electrodes to the plate, and thendiscovering inadequacy of the plate for service as an electricalresonator.

in treating a resonator plate for polarization, the region in the platethat is to be polarized is disposed between two polarizing electrodesfor 4 minutes in mineral oil at a temperature of C., at a voltageadjusted to the value that is at least 100 volts per mil of thickness ofthe plate.

With the procedure of the present invention, full polarizing voltage isapplied, and any deficient condition in the dielectric material, that isbeing treated, is immediately detected by a breakdown in the material,with consequent short circuiting of the applied voltage, to indicate thedeficient nature of the material being treated.

Thus, a further object of the invention is the provision of apiezoelectric resonator whose method of manufacture is such as toinherently assure that the ultimately finished resonator will beadequate and satisfactory in operation.

A further object of the invention is to provide a method of manufacturethat is inherently economical by reason of assuring such earlyelimination of faulty samples, which would represent a substantialeconomic waste if their detection were necessarily deferred until afterthe completion of the manufacturing operation, which involves theexpensive step of applying operating electrodes to the opposite faces ofthe disc.

These and other objects, features and advantages of the invention, andthe manner in which these objects and advantages are obtained, will beapparent to those conversant with the art from the following descriptionand subjoined claims taken in conjunction with the annexed drawings, inwhich:

FIG. 1 is a front view in elevation of a resonator disc with a polingelectrode in place;

FIG 2 is a side elevational view of the disc of FIG. 1, showing bothpoling electrodes;

FIG. 3 is an equivalent circuit diagram of a resonator disc alone, shownin solid line, with an added condenser shown in the broken line circuitto represent the equivalent added capacitance which is added to theresonator disc by the manufacturing procedure and treatment of thepresent invention;

FIG. 4 is a graph illustrating the frequency-spectrumrelationshipbetween the natural inherent series resonance frequency of the resonatordisc and the natural inherent antiresonance frequency of the untreatedresonator disc, and shows how the antiresonance frequency is shifted tothe broken line position in the resonator when the disc is treated andconstructed according to this invention;

FIG. 5 is a plan view of a circular disc of piezoelectric material, andshows the central inner poled area of the region that is polarizedaccording to this invention, and the enlarged electroded area whichincludes an additional annular space beyond the polarized region, butnot extending to the periphery of the ring, at which periphery anuncovered annular border portion is left uncovered to permit unimpedededge vibration of the disc during operation;

FIG. 6 is a front elevational view of a resonator disc of thisinvention, as ultimately electroded, and mounted within its sealedenclosing housing, with the front wall removed to show the constructionof the base and its support for the disc;

- FIG. 7 is a vertical sectional view, taken along the line 7-7 of FIG.6, as a view from the side of the assembled structure of F IG. 6, cutopen to show the disposition and structure of the inside of the housingand the disposition of the disc;

and FIG. 8 is a downward view in section taken along the line 8-8 ofFIG. 7.

Generally speaking, the present invention describes a piezoelectricresonator, as modified to increase its normal capacitance parameter bypolarizing a portion of the resonator within the area usually covered bya working electrode. As a consequence, the effective capacitance of theresonator plate is increased beyond the normal value of that parameterfor an untreated plate, and the antiresonance frequency of the resonatorplate due to its inherent parameters is decreased, the coupling factoris thus diminished and the bank width of the resonator during operationis diminished.

The manner in which the resonator of this invention is treated andconstructed to achieve the improvement may be more readily understoodupon referring to the drawings.

Referring to FlGS. 1 and 2, a piezoelectric resonator disc 12is-disposed between two circular poling electrodes 14 and 16 coaxiallyapplied to the opposite faces of the disc 12. The two electrodes 14 and16 are connected to two leads 18 and 20 for connection to an externalelectric circuit, through which a high voltage is applied to theelectrodes to polarize the volume of the disc 12 between the electrodes,and to establish two fixed poles of opposite polarity on opposite facesof the disc. Those two fixed poles represent bound charges which add anartificial increase to the capacitance parameter of the disc.

In FIG. 3 the elements shown in solid line represent the equivalentcircuit diagram of the parameters of the conventional piezoelectric discshown in FIGS. 1 and 2, before polarizing treatment, and the twoterminals 22 and 24 to represent their electrical equivalents.

In the series circuit between the two terminals 22 and 24 of the diagramin FlG. 3, the inductor L represents the vibrational mass of theresonator, C represents the compliance of the plate, and R representsthe motional loss of the plate. The parallel capacitor C represents thenatural intrinsic dielectric capacitance of the resonator plate. Thenatural resonant frequency is controlled by the motional resonance, atthe value where the reactance of the mass L is equal but opposite to thereactance of the series capacitor C, and represents a series resonantcondition. When the frequency is greater than that resonant frequency,the inductive reactance increases so that the series branch is inductiveand then at an appropriate resonance value will resonate with theparallel capacitance C,,, which then represents the parallel resonancecondition.

Thus far, that is all prior art and represents the usual prior artoperation of a resonator.

Referring now again to FIG. 3, if the dielectric capacitance of theresonator is increased by the addition of capacitance represented by Cas indicated by the broken line circuit, to establish additionalcapacitance in parallel to C the natural dielectric capacitance of theplate, the parallel or antiresonance frequency will be diminished to avalue less than the natural antiresonance frequency of the plate inwhich only the normal dielectric value of capacitance C was available asan operating parameter.

In FlG. 4, the solid-line graph shows the natural impedance relationshiprelative to frequency over the short range between natural resonantfrequency and natural antiresonant frequency. The broken line graphrepresents the modified shifted curve to shift the antiresonantfrequency toward resonant frequency, and thus diminish the spacingbetween them that represents coupling.

FIG. 5 shows a disc 12 with the relative areas indicated for the polingarea 32 which is to be polarized, and the electrode area 34 which is tobe ultimately covered by an electrode on each opposite face of the disc12 after the polarized condition has been established in the area withinthe circle 32 on each face of the disc. Thus, the area covered by theelectrode 34 exceeds the area that is polarized within the circle 32. Anouter annular region 36 is left unpolarized and uncovered by theelectrode and provides an unimpeded region that is free to adjust itselfin response to the vibrations at resonance frequency.

FIG. 6 shows the fully electroded disc 12 with the electrodes 38, one oneach major surface ofthe disc 12.

The disc 12, when provided with the full electrodes 38 as in FIG. 6, isthen mounted to be supported between two fingers 22 and 24 of thestanchions 18 and 20 with the two fingers 22 and 24 engaging theresonator disc coaxially by engagement with the two electrodes 38. A cap45 is fitted over the base 47 to provide a complete enclosure as ahousing for the resonator. Two posts 52 and 54 are shown as integrallyformed on the base 47 and are separated to define a space to accommodatethe resonator plate 12, and they serve also as support for the two upperfingers 22 and 24 which engage and press against the two sides of theelectroded disc 12 to hold the disc in coaxially aligned position.

As shown in H6. 8, the disc is supported by the two fingers 22 and 24engaging the two electrodes 38 and 40 coaxially at the node points ofthe disc 12 for substantially single point support axially, with maximumfreedom of vibration available to the disc without any impediment fromany of the other supporting structures.

Thus, as disclosed herein, the invention includes the primary step ofpoling or polarizing a central region of the disc, over an area that isless than the full operating electrode area of the disc. A feature ofthis preliminary polarizing operation at relatively high voltage is thepossibility of causing breakdown in poor dielectric material or in anyimproper mixture of the piezoelectric material, which permits detectionof a faulty or defective piezoelectric disc before it has been fullyprocessed through all steps of manufacture, and then found to bedefective. Such preliminary detection of a defective disc is asubstantial factor in the economy of the process of manufacture inaccordance with the present invention.

Moreover, this procedure of polarizing and poling a central region ofthe disc after the material of the piezoelectric disc has been fired,provides a new took for treating a fired disc after such firing hasordinarily completed any control over the material while it is beingprepared and formed as a piezoelectric disc.

A further feature of the invention is that this step of poling orpolarizing a region of the material, by varying the ratio of the poledarea to the electrode area, permits the discs from one batch of basicmaterial to be variably treated to make a resonator available forvarious applications where different frequencies are desired, or wheredifferent coupling factors are desired.

Since the poling treatment can be applied independently of the nature ofthe material which enters into the piezoelectric disc, it will beobvious that various ratios between the poling area and the electrodearea may be utilized to control the operational characteristics of thedisc when finally formed. As indicated in the foregoing description, areduction in the coupling and therefore the frequency range ofoperation, is related to the ratio of the parallel resonant frequency tothe series resonant frequency.

It will also be realized that when a portion of the disc is poled, anddriven electrically, it must activate or vibrate the entire mechanicalstructure. Therefore, the dimensions of the complete resonator willdetermine the resonant frequency.

When the larger electrodes are applied for operation, an addedcapacitance is placed across the natural inherent capacitor C in FIG. 3.With the increased capacitance, the parallel resonant frequency islowered. Consequently, for a given physical structure, having acorresponding series resonant frequency, a decrease in the parallelresonant-frequency will result in the decrease of the planar couplingfactor. This added capacitance is determined by the passive section ofthe resonator, that is, that portion of the material contained betweenand covered by the final electrodes, which is unpoled. i

The treatment and construction of the disc, and the ratio between poledand electroded areas, may be modified and varied without departing fromthe spirit and scope of the invention, as particularly defined in theclaims.

. Whatlclaim is:

l. A piezoelectric resonator comprising a piezoelectric 2 wafer having asubstantially uniform thickness between two opposite faces, a centralaxial polarized thickness region and a peripheral unpolarized thicknessregion with each of said regions having a capacitance; and an electrodemeans on each face for electrical connection to an external circuit forenergizing said resonator, said electrode means covering all of thesurface area overlying the polarized capacitance region and at least apart of the unpolarized capacitance region and connecting said regionselectrically in parallel to increase the capacitance and thereby reducethe coupling factor of the resonator.

2. A piezoelectric resonator according to claim I, wherein saidelectrode means are oppositely opposed and of substantially the samesurface area.

3. A piezoelectric resonator according to claim 1, wherein saidelectrode means are oppositely opposed and of substantially the samesurface area.

4; A piezoelectric resonator according to claim 1 further including anelectrical terminal including finger means for resiliently engaging eachof said electrodes, a base means including post means for supportingsaid finger means, and a cap means enclosing said resonator, base meansand finger means to provide a housing therefor.

2. A piezoelectric resonator according to claim 1, wherein saidelectrode means are oppositely opposed and of substantially the samesurface area.
 3. A piezoelectric resonator according to claim 1, whereinsaid electrode means are oppositely opposed and of substantially thesame surface area.
 4. A piezoelectric resonator according to claim 1further including an electrical terminal including finger means forresiliently engaging each of said electrodes, a base means includingpost means for supporting said finger means, and a cap means enclosingsaid resonator, base means and finger means to provide a housingtherefor.